The present invention relates to an underspeed actuator unit which is responsive to operating speed of a prime mover or engine in order to automatically adjust a variable torque transmitting coupling in a transmission driven by the prime mover. More particularly, the underspeed actuator is operable to adjust displacement of a hydrostatic pump only while the prime mover is set for full throttle operation for greater efficiency.
Hydrostatic transmissions adapted for track-type vehicles commonly include parallel pump units each having variable displacement in order to permit operator control over vehicle speed and/or direction by appropriately varying displacement of one or both pumps. The present invention is particularly described below in connection with a hydrostatic transmission of this type. However, it will be apparent from the following description that the present invention is also applicable to more conventional hydrostatic transmissions having a single pump unit or multiple pump units whose displacement is simultaneously varied.
In most hydrostatic transmissions, it is desirable to reduce the load placed upon thee prime mover by the transmission when operating speed of the prime mover falls below a selected range because of an external load, such as when a vehicle including the prime mover and transmission is operating uphill or otherwise encounters an increased load.
The use of underspeed regulator or actuator valves is known in connection with hydrostatic transmissions for overcoming this problem as may be seen by reference to U.S. Pat. No. 3,477,225 and U.S. Pat. No. 3,727,628, both of which are assigned to the Assignee of the present invention. An underspeed actuator similar to that employed by the present invention is also disclosed by a copending application entitled: UNDERSPEED ACTUATOR FOR HYDROSTATIC TRANSMISSION filed on Feb. 20, 1975, as Ser. No. 551,474, assigned to the Assignee of the present invention.
A problem commonly encountered by underspeed actuators of the type disclosed by the above-noted references is that displacement of the hydrostatic pump or pumps is automatically reduced when engine speed is deliberately cut back by manipulation of the throttle. Thus, the variable displacement pump or pumps cannot be maintained at their maximum displacement position when the engine is being operated at part throttle. The problem arises primarily in that efficiency of the hydrostatic transmission is decreased under such circumstances.